The use of epoxy resins as casting resins and molding compounds is well known. In such application, the resins desirably have certain of the properties generally shown by higher molecular weight resins, i.e., are non-sintering, flakable solids, but also have the lower equivalent weights and low melt viscosities characteristic of lower molecular weight resins. It will be appreciated that resins having such a combination of properties are not easily come by; a compromise is generally made.
Conventional expedients include adducting novolacs per se with the diglycidylether of bisphenol A (DGEBA) and the use of ring-substituted epoxy novolacs, such as--for example--"epoxidized" cresol novolacs. The novolac components of such adducts or substituted epoxides consist predominantly of linear, oligomeric molecules having an average of from about 3 to 6 phenolic hydroxyls--as such or as glycidyl ethers thereof.
DGEBA, a low molecular weight diepoxide, is conventionally "advanced" to a higher molecular weight through adduction of oxirane groups with phenolic hydroxyls in novolacs or bisphenols (Bisphenol-A or "bis A", predominantly), thereby producing polyether polyhydroxy epoxides. The adduction conventionally is carried out in the presence of a catalyst or initiator constituting about 0.1 wt.% of the reaction mixture. Typically, the epoxide to bisphenol weight ratio is about 3:1 and the phr of the phenol (parts bisphenol per hundred parts of the "resin" or epoxide) is about 100/3.times.1=33.3. The reactants and catalyst are heated rapidly to onset of an exotherm, at about 150.degree. C. The exotherm is allowed to drive the temperature to a peak value of about 180.degree.-200.degree. C. and then to subside until the temperature reaches about 160.degree. C. This temperature is maintained for about 3 hours and the product is poured into trays and allowed to cool and solidify or is "flaked" (cooled and solidified in the form of chips or flakes).
The resulting advancement products are generally prone to sinter if they have softening points of about 60.degree. C. or less.
Not all conventional molding or casting epoxies are solids and, of those which are, not all are flakable and/or non-sintering. That is, the inconveniences involved in using resins which can't be flaked and which "sinter" or "block" (lose particulate form by fusing under their own weight at ordinary temperatures) are accepted as a trade-off against the higher costs and/or poorer heat resistances (when cured) of more easily worked-with resins. Another trade-off against cost is slower gellation. That is, the lower functionality resins, although slower gelling, are also less expensive and may be acceptable on this account in some specific applications.
Thus, the ordinary, unsubstituted novolac epoxides have acceptably lower melt viscosities and relatively high functionalities and glass transition temperatures (Tg's) but cannot be utilized simply as crude reaction mixtures which have not been worked up (working up costs money). They also sinter, by reason of unavoidably including excessive proportions of low molecular weight species. The solid epoxies obtained by advancement of DGEBA with bisphenol A can be used simply as fusion mixtures but will have high melt viscosities, low Tg's and lower functionalities when of sufficient average molecular weight to be non-sintering. The substituted novolac expoxides have high Tg's, do not include excessive proportions of low molecular weight species and have relatively high functionalities. However, they cannot be used as reaction mixtures which have not been worked up (to remove solvent, by-products; etc.) and have somewhat high melt viscosities.
A recently developed type of epoxy "novolac" is the oligomeric, high functionality epoxides disclosed in U.S. Pat. No. 4,394,496, as being formed by the adduction of 1,1,1-tri(hydroxyphenyl)alkanes with the corresponding triglycidyl ethers thereof. These epoxides have high Tg's and are flakable and non-sintering but also have high melt viscosities, are non-linear and would appear to be useful in epoxy casting or molding resin systems only in admixture with other types of epoxies.
The monomeric triepoxides also disclosed in the patent include some (the triepoxide of leucaurin, most notably) which are liquids of low equivalent weight and--when cured--have high heat distortion temperatures. However, they are also highly viscous.